1. Field of the Invention
The present invention relates to a loop transmission system which includes a plurality of loop-connected node stations, each station including two loop-back circuits, and, more particularly, to a token-passing type loop transmission system without a supervisory loop control apparatus therein, which can detect and identify a fault point therein and can construct a loop transmission line not including the fault point. That is, the present invention relates to a loop transmission system having an automatic loop (or link) configuration control (ALCC) means.
2. Description of the Related Art
Recent factory automation systems, laboratory automation systems, office automation systems, and the like make use of advanced communication systems including a plurality of processors and a plurality of terminal devices suitably connected in accordance with system conditions for optimum distribution of tasks and/or optimum formation of communication paths. The so-called "local area network" (LAN) system represents a typical communication system with loop (or link) connection type transmission.
Generally, a loop transmission system includes a plurality of node stations loop-connected one by one by a communication line, for example, an optical fiber cable, and a plurality of terminal devices and a plurality of processors which may be connected to corresponding node stations. The node stations connect the terminal devices or the processors to the communication line or disconnect the terminal devices or the processors from the communication line.
The node stations are chain-connected. Accordingly the loop transmission system can fail completely if one of the node stations fails or the communication line breaks down, resulting in a breakdown of the loop path. In order to overcome this problem, two communication lines may be provided: a master loop line and sub-loop line for connecting adjacent node stations. There may also be provided master (or an upper) and slave (or a lower) loop-back circuits in each node station. In a normal condition a loop-back circuit is connected to the separate master and sub-loop lines introduced therein for passing signals on the lines. In a non-normal condition, however, the loop-back circuit, for example, the master loop-back circuit, connects the master loop line and sub-loop line to form a loop-back ON state and, accordingly, disconnects (separates) the node station in question and an adjacent node station positioned at a preceding position thereof. As a result, on the one hand, a transmission signal on the main loop line output from the preceding node station cannot reach the node station in question. On the other hand, a transmission signal on the sub-loop line output from an adjacent node station positioned at a following position thereof can be returned to the following node station through the main loop line connected to the sub-loop line at the master loop-back circuit. This helps transmission loop reconfiguration in the loop transmission system.
In the prior art, a known loop transmission system includes a supervisory station for reconfiguring the loop construction (for example, EPC Publication No. 0102222, Mar. 7, 1984). The loop transmission system, however, requires an extra supervisory loop control station and suffers from disadvantages of low availability in the case of failure of that supervisory loop control station, a low speed of reconfiguration of the loop and high cost.
There is also a known token-passing system without a supervisory station, for example "A Token-Ring Architecture for Local-Area Networks," by D. W. Andrews and G. D. Schultz, Mar. 8, 1982, IEEE Project 802 on Local Area Networks. In this field, "token" means polling information. The loop transmission system of the present invention pertains to this type of loop transmission system. However, the token-passing systems of the prior art suffer from the disadvantage of low availability and difficult system maintenance.